LVIV, Ukraine, Feb 19 (Reuters) - Opponents of Ukraine's president declared political autonomy in the major western city of Lviv on Wednesday after a night of violence when protesters seized public buildings and forced police to surrender.

Raising the prospect of Ukraine splitting along a historic cultural and linguistic faultline, the regional assembly in Lviv, a bastion of Ukrainian nationalism near the Polish border, issued a statement condemning President Viktor Yanukovich's government for its "open warfare" on demonstrators in Kiev and saying it took executive power locally for itself.

In other signs of fraying central control for a government seen as close to business magnates from the Russian-speaking east, Poland said Ukrainians blocked the Korczowa border crossing near Lviv. And local media said opposition groups in other western cities, including Khmelnitsky, Ivano-Frankivsk, Uzhorod and Ternopil, also took over public buildings.

Overnight in Lviv, an ancient city of 750,000 and capital of a region of 2.5 million, hundreds of people took part in protests as demonstrators fought riot police in central Kiev, nearly 500 km (300 miles) to the east.

Young men in ski masks seized the offices of Yanukovich's administration in Lviv and forced a surrender by Interior Ministry police, making officers come out with their hands up.

The developments, in a region traditionally hostile to the easterner Yanukovich and fiercely in favour of closer ties with the European Union, drove home the sense of a country, or at least part of it, slipping from his grip.

It underscored, too, the dangers facing a divided Ukraine, its 46 million people trapped in a geopolitical tug-of-war between Russia and the West.

RAMPAGE

From late on Tuesday, taking their cue from the drama in Kiev that was Ukraine's bloodiest day in just over 22 years of post-Soviet independence, mobs swept through Lviv's picturesque city centre in a spasm of violence that went unchallenged.

Erecting barricades outside a police barracks, the protesters demanded their surrender. Officers filed out, hands above their heads, and were stripped of their body armour.

The barracks were set ablaze, as were the office of the state security service and the premises of the state prosecutor. Rioters tossed papers through smashed windows.

Torched cars lay upturned in the streets.

"We are the citizens of Lviv," the crowd chanted. "We are the strongest."

The regional parliament in Lviv accused Yanukovich's government of "open warfare against the people".

"The parliament takes upon itself all responsibility for the future of the region and its people," it said in the statement.

Though many leaders on either side of the confrontation in Ukraine play down cultural and linguistic tensions, there are marked contrasts between east and west.

Lviv's baroque and neoclassical architecture recalls its past as a regional capital of the Austrian Habsburg empire. Between the world wars, it was Polish. By contrast, Soviet concrete marks the cities of eastern Ukraine, home to coal and steel industries and historically ruled by the Russian tsars.

Yanukovich's political base, many people in the east speak Russian as their first language and prefer to maintain old connections with Moscow. Many western Ukrainians, eager for closer ties with the European Union, accuse Yanukovich of being a Kremlin stooge and mastermind of corruption on a grand scale.

Ukrainian television said that in Khmelnitsky protesters had seized the regional administration building and had attacked the headquarters of the state security service.

Police said protesters had also seized the administration building in Ivano-Frankivsk, to the southeast of Lviv. Media reports said the main police station in Ternopil was set ablaze. In the east of the country, cities were calm.

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